Put your diced bacon in a 4-5 qt stockpot or Dutch oven over medium high heat and cook until the bacon bits are crispy and you've rendered out as much fat as possible.
1/4 lb thick-cut bacon
Remove the bacon bits and set them aside.
Add the onion to the pot and turn up the heat slightly. Cook the onion until it becomes translucent and then starts to brown and caramelize a little.
1 med onion
Add in the garlic and give it a quick stir for 2-3 minutes.
3-4 cloves garlic
Add the collard greens. The greens will fill the pot entirely, but keep tossing and stirring them until they're well coated with the oil in the pot, mixed with the onion and garlic, and starting to soften and turn a deeper green.
1 lb collard greens
Once the collards have started to wilt down a little, maybe 5 mins, add the piquillo peppers with their juice, the diced tomatoes and juice, and the vinegar.
1 8oz jar piquillo peppers, 1 15oz can diced tomatoes, 1-2 tbsp apple cider vinegar
Finally add enough stock or other liquid to come to about halfway up the level of the greens - around 4 cups. Keep in mind that the greens will shrink as they cook and you don't want too much liquid.
4 cups ham stock
Lower the heat to a simmer and cook the greens stirring occasionally, for around 4 hours, minimum. You want them to be tender but not mushy.
Serve with bacon bits sprinkled over the top or with a dash of pepper vinegar.